The present invention generally relates to transmitters and receivers, and more particularly to a transmitter which employs a coded modulation technique and to a receiver adapted to receive information from such a transmitter.
In digital radio communication systems, an M-ary modulation scheme is in many cases applied to a radio transmission channel. Particularly in the case of a mobile communication system, proposals have been made to apply a coded modulation technique employing the M-ary modulation scheme in order to simultaneously realize reduction in the power consumed at a mobile station and effective utilization of finite radio frequencies.
FIG. 1 is a system block diagram showing an example of a transmitter employing the coded modulation technique which is called trellis coded modulation (TCM) or block coded modulation (BCM) scheme. In FIG.1, transmission information is divided into n-bit blocks. 1 bit of each n-bit block is input to an M-ary modulator 165, while the remaining n-1 bits are input to the M-ary modulator 165 via an encoder 166. A transmitting wave signal is obtained from an output of the M-ary modulator 165, and this transmitting wave signal is supplied to a transmitting part which is not shown.
In the transmitter having the construction shown in FIG.1, the encoder 166 generates a code sequence by subjecting the n-1 bits of each n-bit block to a trellis coding (or block coding) at a rate R of (n-1)/n. The M-ary modulator 165 employing (n+1)-bit level reads the n-bit code sequence and the 1 bit which is not encoded, and generates the transmitting wave signal by subjecting a carrier wave signal to a (n+1)-level modulation based on the two.
For simplicity, if it is assumed that an 8-phase phase shift keying is employed, signal points of the transmitting wave signal which is generated in the above described manner become as shown in FIG. 2 by the M-ary modulator 165 by mapping symbols a.sub.1, a.sub.2 and a.sub.3 (n=2 in this case) based on the set partitioning. Accordingly, the symbols a.sub.1, a.sub.2 and a.sub.3 and the signal points are set to values which satisfy a corresponding relationship C.sub.1 .DELTA..sub.1 =C.sub.2 .DELTA..sub.2 =C.sub.3 .DELTA..sub.3 when a uniform error protection is possible, where C.sub.1, C.sub.2 and C.sub.3 denote minimum distances of the codes for each bit level, respectively, and .DELTA..sub.1, .DELTA..sub.2 and .DELTA..sub.3 denote minimum distances between the signal points in the signal space. In this case, assuming a non-coded bit is a.sub.1, C.sub.1 .DELTA..sub.1 =C.sub.2 .DELTA..sub.2 is required because of C.sub.2 =C.sub.3.
Furthermore, at a receiving end which receives and demodulates the transmitting wave signal described above, the structure of the trellis diagram becomes simple because the received signal to be demodulated is given by a single coding level, and a maximum likelihood decoding based on Viterbi decoding can be carried out efficiently.
FIG. 3 is a system block diagram showing another example of the transmitter employing the coded modulation, which is called a multi-level coded modulation (MLCM) scheme. In FIG. 3, the transmission information is input to a serial-to-parallel converter 170, and M outputs of the serial-to-parallel converter 170 are input to a mapping part 172 via corresponding encoders 171.sub.1 through 171.sub.M. An output of the mapping part 172 is input to a modulator 173, and the transmitting wave signal is obtained from an output of the modulator 173.
In the transmitter having the construction shown in FIG. 3, the serial-to-parallel converter 170 divides the transmission information into units of M bits and carries out a serial-to-parallel conversion. The encoders 171.sub.1 through 171.sub.M independently encode the M groups which are obtained in parallel from the serial-to-parallel converter 170 at a desired coding level. The mapping part 172 and the modulator 173 modulate a carrier wave signal depending on each of the block codes which are generated by the encoding, so as to generate the transmitting wave signal.
The corresponding relationship of the signal points of the generated transmitting wave signal and the symbols corresponding to the signal points is similar to that of the example shown in FIG. 1, and a description thereof will be omitted.
According to the transmitter shown in FIG. 3, the encodings at the individual bit levels are carried out in parallel, and the rate of the encoding can positively be set to a desired value. For this reason, it is possible to secure the minimum value of the distance between the signal points of each individual group (bit level) to a higher value as compared to the example shown in FIG. 1.
The asynchronous transfer mode (ATM) is a transmission system for realizing mainly a broadband integrated services digital network (B-ISDN), and the precondition is to make the transmission by wire, particularly by optic fiber. Hence, it is a precondition that the bit error rate (BER) in a satisfactory state is 10.sup.-11 or less.
In general, the required BER of the header is 10.sup.-7 to 10.sup.-11 or less since the cell loss rate affects the system performance, and the required BER of the data is 10.sup.-6 or less for images or the like.
On the other hand, the BER performance of the radio communication is poor, and the radio communication is mainly used for voice transmission with a required BER of 10.sup.-2 or greater and for low-speed data on the order of several kbps. Furthermore, in the mobile communication systems, the BER performance is floored due to multipath fading.
In such a small channel capacity, it is not effective, both frequency-wise and power-wise, to make the overall BER performance to a very small value less than or equal to 10.sup.-11, for example. For this reason, in order to realize the ATM in the radio communication, it is necessary to prepare 2 different channels for the header and the data, respectively, so that the required BER for the header is 10.sup.-7 to 10.sup.-11 or less and the required BER for the data is 10.sup.-6 or less. The BER performance for the header is set very small because control information such as destination information is included in the header, and the cell cannot be received if the contents or sequence of the headers are damaged or changed due to the error. The cell which cannot be received must be discarded, and then the system performance is heavily degraded.
On the other hand, in order to make a high-speed data transmission in a channel having a poor BER performance, it is essential to employ an error correction technique including coded modulation schemes, and such an error correction technique has come into practical use in satellite communications and some mobile communications. In addition, as techniques for compensating for the fading, it is known that a diversity technique, an adaptive antenna technique using directional antenna, and equalization are effective in eliminating the BER floor.
However, with respect to the conventional trellis coded modulation, block coded modulation, and multi-level coded modulation using the non-coded bit level, a large number of bit errors are generated for the non-coded bit level due to the fluctuation of the transmission characteristic of the radio transmission channel, such as fading, and the overall transmission performance gets a few gain.
On the other hand, according to the multi-level coded modulation not using the non-coded bit level, or using more than one coding level, the trellis structure becomes more complicated as the number of coding levels increases and for this reason, a multi-stage decoding is carried out at the receiving end. In other words, decoders D.sub.1 through D.sub.M sequentially carry out the decoding process under timings determined by the multiple stages of delays provided by delay elements T.sub.1 through T.sub.M, and results of the decoded process are subjected to a parallel-to-serial conversion in a parallel-to-serial converter PSC as shown in FIG. 4. According to this multi-stage decoding, a decoding delay equal to a sum of the delays provided by the delay elements T.sub.1 through T.sub.M occurs, thereby degrading the real-time transmission performance. Furthermore, since the coding level in the higher layer cannot use the decoded result of the coding level in the lower layer, a maximum likelihood decoding cannot be achieved and the performance is degraded.
In addition, in the ATM network by use of the cells, the transmission must wait until a predetermined amount of information is filled in the cells. Hence, in such an ATM network, a transmission delay occurs when transmitting low-speed data such as voice data because it takes time for the information to be filled in the cells. On the other hand, when transmitting high-speed data such as image data, a transmission delay similarly occurs in the process of carrying out error correction and re-transmission (ARQ: automatic repeat request) in order to meet a required BER.
Furthermore, in order to efficiently realize the radio communication employing the ATM, it is conceivable to prepare 2 different communication systems with different BERS, that is, one for the header with a BER of 10.sup.-7 to 10.sup.-11 or less and another for the data with a BER of 10.sup.-6 or less. However, the efficiency becomes poor if 2 physically different channels are prepared, individually.